| Long-term shifts in the abundances of open ocean fishes have been observed over the period of 1996-2011 in the North Pacific Subtropical Gyre (NPSG). Fishery data indicate that large, high trophic position fishes such as tunas, billfishes, and sharks, have been decreasing in relative abundance, while smaller, mid-trophic position fishes such as alepisaurids and gempylids have been increasing. Some of these species are of great commercial importance, and recent calls for ecosystem-based fishery management require accurate characterization of predator-prey relationships supporting biomass production at the top of the food web. The broad goal of this study was to elucidate pelagic food web structure within the NPSG, focusing on the links formed by understudied mid-trophic level animals (micronekton and their predators), and how energy is transferred through feeding interactions between these animals inhabiting different vertical habitats. Four distinct but complementary approaches were employed: stomach content analysis, bulk stable isotope analysis, compound-specific nitrogen isotope analysis of individual amino acids (AA-CSIA), and total mercury concentrations.;Stomach content analysis revealed that pelagic fish species that have been increasing in abundance have distinct diets relative to adult tuna and billfishes. Unique components of the micronekton community were exploited by alepisaurids, gempylids, and lamprids, including hyperiid amphipods, the little-known cephalopod Walvisteuthis youngorum, and mesopelagic fishes (Anoplogaster cornuta, Sternoptyx spp.), and the polychaete Vanadis nans. Despite shared vertical habitat and unique diets, AA-CSIA indicated that both groups of fishes have similar trophic positions. Thus, shifts in abundances could be the result of predation release since tunas and billfishes feed upon smaller size classes of alepisaurids and gempylids.;Biochemical metrics of trophic position (bulk tissue delta15 N values, total mercury concentrations, and AA-CSIA data) were strongly related to animal size for diverse food web components ranging from zooplankton to micronekton to large pelagic fishes. Distinct increases in the delta 15N values of glycine, serine, and phenylalanine were observed with micronekton fish depth of capture, suggesting a separation in food resources across epipelagic, mesopelagic, and upper bathypelagic habitats. Broadly, this study established that the pelagic food web of the NPSG is structured by size-based interactions and the influence of vertical habitat. |
